Labyrinth might be described as the thinking woman's summer reading, chick lit with A levels for those with only a passing interest in getting a boyfriend. The story - a quest narrative set simultaneously in the 13th century and the present - concerns the safety of a set of crumbly books containing eternal verities that date back to ancient Egypt. The books belong to the Cathars, a sect active in medieval Pyrenean France who represent tolerance, ecumenicalism and all things nice (in fact, at times they resemble nothing so much as go-ahead Church of England curates in a multicultural urban parish). The baddies are the northern French, who disguise their greedy designs on the rich agricultural land of the Languedoc by using the language of religious orthodoxy and the cultural authority of the inquisition.

What really marks Labyrinth out is the fact that all the main roles - goodies as well as baddies, historical and contemporary - go to women. The job of heroine is split between the 13th-century Alaïs, the daughter of one of the leading men in Carcassonne, and Alice, a contemporary British woman who has managed to inveigle her way on to a local archaeological dig as a way of spinning out a lacklustre summer. As Alaïs and Alice become increasingly involved in preventing the books falling into the wrong hands, their adventures echo, qualify and extend one another's as if they were playing the same orchestral piece on slightly different instruments. It is a testimony to Kate Mosse's control over her material that the two narratives never seem to repeat or collide or, indeed, swamp one another.

The baddies, meanwhile, include Marie-Cecile, a French businesswoman who is after the books and who you know is evil because she wears sophisticated linen two-pieces that never seem to crease, despite the sweltering heat. The other wicked woman is Alaïs's sister, Oriane, who is not only raven-haired and curvy, in contrast to her sister's airy fairness, but also sleeping with her husband, Guilhem (and not for nothing is Marie-Cecile's part-time boyfriend, who turns out also to be an old contact of Alice's, called Will). As these examples suggest, Mosse's writing stays firmly within the conventional paradigms of popular historical fiction. At one point Alaïs's hair is described as being like a "waterfall", while Marie-Cecile's eyes resemble a cat's (if, in real life, you stumbled across such peculiar phenomena you would, depending on your sense of adventure, call either the emergency services or the local newspaper at once).

However, it looks as if consciously confining herself to these rules of engagement has allowed Mosse's imagination to leap in other ways. Labyrinth is saturated with a passionate understanding of the region's past in a way that puts more conventional historical accounts to shame. Virginia Woolf once wrote of wishing that she could push a plug into the wall of various places where she had lived as a way of accessing all those layers of drama and conversation that she believed were sealed up there, waiting to be tapped. In Labyrinth Mosse manages, miraculously, to do just that, plugging not simply into the city wall of medieval Carcassonne or into the imagined remains of the castle where Alaïs lives with her father and sister, but into all those cheap, melancholic hotels and over-formal places of business that mark this particular corner of France.

The languages that Mosse channels through her magic plug include both French and Occitan, and she sprinkles her text with them very effectively, simultaneously signalling that the past really is another country, rare and strange, and that, at the same time, it is open to the possibility of being read, understood and learned from. For if you take away the foreground story about the quest for the books, what you are left with is a resonant account of the first European genocide - more than a million Cathars died as a result of what became known as the "Albigensian Crusade" - fuelled by the language of fear and hate dressed up as religious piety.

None of this would matter, of course, if it were a struggle to get through the novel's 500-odd pages. But Mosse wears her learning so lightly, knitting her historical research so neatly into her narrative, that we never get the slightest sense of being preached or lectured to. In this she is reminiscent of those twin goddesses of popular historical fiction, Jean Plaidy and Mary Renault, both of whom managed to convey the texture of various patches of the past with such rich complexity that they were responsible for turning more young women on to history than anyone, including the girls themselves, would probably quite like to admit.

· Kathryn Hughes's biography of Mrs Beeton is published in the autumn. To order Labyrinth for £9.99 with free UK p&p call Guardian book service on 0870 836 0875.